1. Field of the Invention
This invention relates, in general, to reciprocating drive mechanisms for machines requiring reciprocating drives with trapezoidal velocity profiles, and in particular, to reciprocating drive mechanisms for dot matrix printers having hammer banks.
2. Description of the Prior Art
For maximum thruput of a dot matrix printer, the hammer bank must travel in the print region at substantially constant velocity. The magnitude of the constant velocity is set by the maximum frequency the hammers can operate, and the desired spacing between consecutive dots. At the end of the print region, the hammer banks must reverse their direction and reach their set constant velocity in a minimum possible time. In practice the best velocity profile (velocity vs. distance) is trapezoidal in shape, having round corners for vibration free operation. To change velocity profile of a linear spring mass system from sinosoidal to trapezoidal profile, energy must be added to and subtracted from the system in an appropriate manner.
In reference U.S. Pat. No. 3,941,051, issued to Barrus et al., a cam driven system absorbs and returns the energy to the mass by a flywheel. The performance of this system is satisfactory at relatively low velocities of 20 inches per second. At higher velocities of 30 to 40 inches per second, as required by high speed print hammers, a cam driven system requires a large drive motor and flywheel, is susceptible to excessive wear at the cam surface, and it is also susceptible to vibration.
In other types of drivers, the adding and subtracting of energy is performed by large, fast, responsive voice coil type motors aided by linear springs. A driver, as such, consumes formidable amounts of power for achieving a high amplitude trapezoidal velocity profile and requires a very large and expensive type of motor. As an alternative to the large amount of energy addition and subtraction by a linear motor, reference U.S. Pat. No. 4,180,766 issued to J. Matula, presents free traveling hammer banks moving at constant speed in the print region to impact cantilever supported flexures acting as resilient bumpers. Since the cantilever flexures are linear springs, they have linear energy absorption characteristics. They therefore fall short of achieving a high amplitude trapezoidal velocity profile. Disadvantages of this type of drive, especially at high amplitude velocities, are the problems associated with impact such as vibration, high rate of wear at the impacting surfaces, and fatigue. Also a common problem to previous mechanisms is the vibration and deflection caused by forces reacting to accelerating and decelerating mass of the shuttle and its counter mass. These reacting forces transmit bending moments to the base of the machine causing vibration and deflection of critical members.